1. Field of the Invention
The present invention relates generally to the field of optical imaging and more particularly to systems for sub-aperture data imaging of double sided interferometric specimens, such as semiconductor wafers.
2. Description of the Related Art
The progress of the semiconductor industry over the last years has resulted in a sharp increase in the diameters of semiconductor wafers as base material for chip production for economic and process technical reasons. Wafers having diameters of 200 and 300 millimeters are currently processed as a matter of course.
At present manufacturers and processors of wafers in the 200 and 300 mm range do not have a wide range of measuring devices available which enable inspection of particular geometric features, namely flatness, curvature, and thickness variation, with sufficient resolution and precision.
As scanning of specimens has improved to the sub-aperture range, the time required to perform full specimen inspection for a dual-sided specimen has also increased. Various inspection approaches have been employed, such as performing an inspection of one side of the specimen, inverting the specimen, and then inspecting the other side thereof. Such a system requires mechanically handling the specimen, which is undesirable. Further, the act of inspecting the specimen has generally required binding the specimen, which can cause deformation at the edges of the specimen, increase defects at the edge, or cause bending of the specimen during inspection.
One method for inspecting both sides of a dual sided specimen is disclosed in PCT Application PCT/EP/03881 to Dieter Mueller and currently assigned to the KLA-Tencor Corporation, the assignee of the current application. The system disclosed therein uses a phase shifting interferometric design which facilitates the simultaneous topography measurement of both sides of a specimen, such as a semiconductor wafer, as well as the thickness variation of the wafer. A simplified drawing of the Mueller grazing incidence interferometer design is illustrated in FIG. 1A. The system of FIG. 1A uses a collimated laser light source 101 along with a lens arrangement 102 to cause grazing of light energy off the surface of both sides of the specimen 103 simultaneously. A second lens arrangement 104 then provides focusing of the resultant light energy and a detector 105 provides for detection of the light energy.
The design of FIG. 1A is highly useful in performing topographical measurements for both sides of a dual-sided specimen in a single measurement cycle, but suffers from some drawbacks. First, the system requires minimum specimen movement during measurement, which can be difficult due to vibration in the surrounding area and vibration of the specimen itself. Further, the inspection can be time consuming and requires highly precise light energy application and lensing, which is expensive. The specimen must be free standing and free of edge forces, and the incidence geometry during inspection must be unimpeded. Illumination access must be preserved under all incidence angles. These factors provide mechanical challenges for successfully supporting the specimen; excessive application of force at a minimum number of points may deform the specimen, while numerous contact points impede access and require exact positioning to avoid specimen deformation or bending during inspection. Further, edge support of the specimen has a tendency to cause the specimen to act like a membrane and induce vibration due to slight acoustic or seismic disturbances. This membrane tendency combined with the other problems noted above have generally been addressed by including most components of the system within an enclosure that minimizes ambient vibrations, which adds significant cost to the system and does not fully solve all vibration problems.
Further, the previous system has a tendency to require excessive coherence lengths. As is generally known in the art, the coherence length is the distance along the emitted laser beam over which the laser light has sufficient coherence to produce visible interference fringes. Coherence length is important when a laser beam is split and recombined to form an interference pattern, as in the system presented in FIG. 1A.
In general, when a laser beam is split, the optical path difference is the difference in length between the two paths before recombining. If the optical path difference is less than the longitudinal spatial coherence length of the light beam, interference fringes are formed at the receiving element, or screen. If the optical path difference is greater than the longitudinal spatial coherence length, no interference fringes form. Thus it is desirable to have a small spatial coherence length to minimize the size of the components involved.
The system of FIG. 1A provides a high spatial coherence between the reference wave fronts and the specimen wave fronts. Such a system makes the overall measurement system highly sensitive to background noise along the optical path. The noise creates a diffraction pattern on top of the measurement signal and thus degrades the image obtained of the surfaces. In particular, the background signal tends to be unstable and can be difficult to correct using compensation techniques.
The cost of lenses sized to accommodate inspection of a full wafer in the arrangement shown in FIG. 1A is significant, and such lenses generally have the same diameter as the diameter of the specimen, on the order of 200 or 300 millimeters depending on the application. Full aperture decollimating optics, including precision lenses, gratings, and beamsplitters used in a configuration for performing full inspection of a 300 millimeter specimen are extremely expensive, generally costing orders of magnitude more than optical components half the diameter of the wafer.
Further, the system disclosed in FIG. 1A requires a high spatial coherence between the reference wave fronts and the specimen wave fronts, making the system sensitive to background noise along the optical path. Noise creates a diffraction pattern that increases the measurement signal in a random fashion. The result unstable and compensation for the combined effect is extremely difficult.
It is an object of the current system to provide a system having a relatively small spatial coherence length to minimize system sensitivity to background noise along the optical path and permit use of reasonably sized enclosure components.
It is another object of the current invention to provide a system for performing a single measurement cycle inspection of a dual-sided specimen having dimensions up to and greater than 300 millimeters.
It is a further object of the present invention to provide a system for inspection of dual-sided specimens without requiring an excessive number of binding points and simultaneously allowing free access for inspection of both sides of the specimen.
It is a further object of the current invention to provide for the single measurement cycle inspection of a dual-sided specimen while minimizing the tendency for the specimen to behave as a membrane and minimize any acoustic and/or seismic vibrations associated with the inspection apparatus and process.
It is still a further object of the present invention to accomplish all of the aforesaid objectives at a relatively low cost, particularly in connection with the collimating and decollimating optics and any enclosures required to minimize acoustic and seismic vibrations.